Conventional generators for converting mechanical energy into electrical energy in a motor vehicle are available. Claw pole generators are usually used in this connection. According to the existing art, these are usually equipped with electrical excitation. Because claw-pole generators generate alternating current, usually three-phase, rectification is necessary for the usual DC voltage electrical systems of motor vehicles. Rectifiers based on semiconductor diodes are existing art.
Conventional generators that can also be used for vehicle drive purposes (i.e., are also operable in motor mode) are available in, e.g., the hybrid vehicles sector. The objective here is to assist the internal combustion engine at low rotation speeds, at which it cannot yet supply its full torque (boost operation, turbo lag compensation). In addition, active electric braking allows the vehicle's kinetic energy to be fed back into the vehicle's electrical system (regeneration). Permanently excited synchronous machines that are operated at higher voltages (typically more than 100 V) are usually used in this context. This results in a comparatively complex system configuration together with considerable modifications in the drive train as well as comparatively laborious safety measures due to the high voltages.
Conventional rotating field electric machines are also available. These can be embodied as three-phase electric machines having no neutral conductor. The phase currents, used in motor mode, of corresponding rotating field machines or of their stator windings can be defined using vector-oriented control methods (also referred to as “vector control”). The Clarke or Park transformation (or the respective inverse transformations) are utilized for this. For three-phase rotating field machines all three phase currents are uniquely defined respectively via the vectors α and β in the Clarke transformation and the vectors d and q in the Park transformation, since only two phase currents can be freely determined and the third results from summing the other two currents.
Terms such as “rotating field electric machine,” “rotating field electric drive,” and “electric motor” are used synonymously hereinafter. This refers in each case to electric machines operable at least at times in motor mode, optionally also in generator mode, for example for regeneration, whose stator windings in motor mode are commutated via a power converter with currents (phase currents) in accordance with a control application pattern, so that a rotating electric field is formed. The “stator windings” are constituted here by one or more assemblages of electrical coils in, for example, a star configuration or delta configuration. When it is stated below that “phases” are energized or impinged upon with currents, this means that a corresponding current is flowing through the respective stator windings. The aforesaid indications are synonymous therewith.
In order to protect the electric machine, in particular its stator windings, from excessively high temperature, it is desirable to know the actual temperature thereof. Conventional methods without the aid of temperature sensors operate by way of a determination of the resistance of the stator windings. Using the ohmic resistance thereby ascertained, it is possible to determine the temperature of the winding. The measurement accuracy depends, however, on the tolerance of the current measurement, the accuracy of the motor model used for calculation, and principally on the proportion of the ohmic resistance in terms of the total reactance of the machine.
European Patent No. EP 2 421 147 A1 describes a method for the identification, without rotary encoder, of electrical equivalent circuit diagram parameters of a three-phase synchronous motor. Levi et al., IEEE Trans. Energy Convers. 19(3), 2004, 508-517 describe foundations of the operation of a novel multi-phase vector-regulated drive having multiple motors. European Patent No. EP 0 584 615 A1 relates to a measuring device for determining the winding temperature of a three-phase electric machine.
A need therefore exists for improved capabilities for furnishing corresponding values for maximally exact temperature determination without the aid of temperature sensors.